GALVANIZED STEEL - APPLICATIONS AND PROCESSES
GENERAL GALVANIZING PROCESS
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The application of a metallic zinc coating over steel is commonly referred to as galvanizing. This is done by different methods with different properties. The galvanizing process differs in layer thickness and production of the zinc coating and the resulting influence on protection duration and mechanical strength. The most significant procedures are
- Discontinuous hot dip galvanizing (galvanizing part)
- Continuous hot dip galvanizing (galvanizing strips)
- Thermal spray with zinc
Coating systems containing zinc, such as zinc dust and zinc flake coatings, are not among the galvanizing processes, although they are often associated with the term "galvanizing". The common element of all galvanizing is the application of a metallic coating that consists essentially of zinc. Coating systems containing zinc do not meet this criterion. Furthermore, protection against cathodic corrosion is not part of the galvanizing process.
DEFINITION OF GALVANIZATION
Discontinuous hot-dip galvanizing, i.e. hot-dip galvanizing, is the application of a zinc coating to steel or immersion castings of mainly prefabricated work pieces (individual parts) in molten zinc. Zinc coatings produced by galvanizing parts offer complete protection as the components or products already manufactured are galvanized and destruction of the applied zinc layer does not occur by further processing. Piece-galvanized coatings have significantly greater layer thicknesses than coatings produced by strip galvanizing. They therefore achieve significantly longer protection periods.
In hot-dip galvanizing, zinc-mixed steel parts in baths are freed of impurities such as grease, oil, rust and scale, then flux treated, thus hot-dip galvanizing takes place. The plating material is immersed in a liquid bath of molten zinc at approximately 450 °C.
Action behavior / corrosion
In the case of hot dip galvanizing of individual parts, a zinc coating is produced, which is basically one of the barrier inhibiting passive corrosion protection methods. In the galvanizing process, as a result of the mutual diffusion of liquid zinc with the steel surface, a coating of layers of iron-zinc alloy of varying composition is formed. When removing hot-dip galvanized objects, a layer of pure zinc usually remains on the top alloy layer.
The usual thickness of the zinc layer of galvanized coatings per piece is 50 to 150 microns. The metallic coating of a galvanizing part also acts as a sacrificial anode in case of coating failures. Batch produced coatings have the same comparable corrosion rates under normal atmospheric loads as continuously produced zinc coatings or alloy coatings.
Standardization and application areas
The basic standard for galvanizing parts is DIN EN ISO 1461. In addition, there are special product standards. Partially galvanized components and buildings generally achieve protection periods of 50 years or more in atmospheric corrosion explosions. As a result, they are used for almost all outdoor applications. Typical fields of application are steel and metal construction applications as well as traffic engineering, agriculture and mechanical engineering.
GALVANIZING STRIPS (PURE ZINC COATINGS)
Strip galvanizing is a continuous hot-dip galvanizing process, in which steel strip is immersed in molten zinc in a continuous process and further processed. Alternative terms for strip galvanizing include the terms Sendzimir galvanizing and continuous hot dip with zinc or zinc alloys. Zinc galvanizing is described below.
The starting material is usually a strip of cold-rolled, unannealed steel with thicknesses from 0.4 to 3 mm and widths from 600 to 2000 mm or strips cut to various widths from 20 mm to less than 600 mm, which are coil wound. Before the galvanizing process, the steel strip is unwound from the coil and welded together to form an endless belt so that the system can be driven continuously. The actual treatment part consists of the continuous furnace, the molten bath, a device for adjusting the zinc coating and cooling. The strip is subjected to a pre-heat treatment and continuously annealed. In the cooling zone, the tape is cooled under shielding gas to a temperature close to the melting bath temperature, so that it dips into the zinc melting bath with a bath temperature of about 450-460 °C. Through a roller, the track is deflected upwards and leaves the pool molten again. The hot dip coated steel strip is then chemically treated, for example oil wound or phosphated and wound on a coil.
Action behavior / corrosion
In continuous hot dip galvanizing of thin sheets, a hot dip coating is produced, which is basically one of the passive barrier corrosion protection methods. It forms, by diffusion, a very thin iron-zinc alloy layer on which a pure metal layer is formed, which is formed by pulling the tape from the molten bath.
The usual thickness of the zinc layer of galvanized coatings is 5 to 25 microns. The metallic coating of a galvanizing strip acts as a galvanizing part and also acts as a sacrificial anode in case of damage to the coating. With steel sheet continuously galvanized with pure zinc coatings, corrosion rates comparable to galvanized coatings can be assumed. Due to the significantly thinner zinc layer thickness of these coatings, however, a significantly shorter protection duration of strip galvanized coatings results in the same corrosion rate.
Standardization and application areas
Strip galvanizing is regulated in DIN EN 10143 and in DIN EN 10346. Galvanized steel is a semi-finished product, that is, an intermediate product that is further processed after galvanizing through molding, stamping and cutting. As a result, the protective layer of zinc is destroyed at the cutting and punching edges. Galvanized steels are typically used in areas with low levels of corrosion. Cable ducts or air conditioning elements are typical examples of this. Strip galvanized steels are generally out of the question for outdoor use because they do not reach the normally required protection periods of several decades due to the thin layer thickness.
STRIP GALVANIZING (ALLOY COATINGS)
A variant of strip galvanizing is the so-called zinc alloy hot dip, in which zinc alloy coatings are applied to the steel strip. These consist of zinc aluminum (ZA), zinc magnesium (ZM) or zinc aluminum magnesium (ZAM). The usual layer thickness of these coatings is up to 25 microns. THE process sequence in hot dip coating with zinc alloys largely corresponds to hot dip coating with zinc and is described in " Galvanizing strips (pure zinc coatings) ". Also with zinc alloys, melt-refined steel strip is a so-called semi-finished product, which is a precursor, which is further processed by galvanizing by stamping, stamping and cutting. As a result, the protective layer of zinc is destroyed at the cutting and punching edges.
Zinc-aluminium, zinc-magnesium and zinc-aluminium-magnesium galvanized coatings reveal in short-term tests. As in the salt spray test, better corrosion resistance compared to zinc galvanized coatings or galvanized coatings. This increase in corrosion resistance may so far not be proven under normal natural outdoor weathering conditions. For example, natural weather tests lasting 10 years did not show different corrosion rates between safety barriers with pure zinc coatings and those with zinc-aluminium coatings. A tunnel exchange test also did not show different corrosion rates between pure zinc coatings and zinc-aluminium-magnesium coatings.
Under much higher corrosion loads, such as in offshore applications, a slight advantage of zinc-aluminum-magnesium alloy coatings can be seen, but this is much less than what is shown in some manufacturers' brochures. There is talk of "up to 10 times better" speech. In classic construction applications in rural or urban climates (corrosivity categories C2, C3, C4), the results of short-term laboratory tests such as the salt spray test could not be confirmed. In this case, alloy coatings produced by strip galvanizing have corrosion rates comparable to pure zinc coatings produced by strip galvanizing or parts galvanizing. The duration of protection is thus determined not by the nature of the zinc coating, but by the thickness of the zinc coating.
Alloy coatings made of zinc and aluminum are shown as pure zinc coatings from hot dip galvanized steel in DIN EN 10346. Zinc-aluminium-magnesium coatings are not regulatory regulated.
For example, while zinc-aluminium coated steel strips are used in construction as thin-walled facade and additionally coated wall elements, no typical uses for zinc-aluminium-magnesium coatings can currently be defined. Zinc-aluminium-magnesium coatings are often in competition with galvanized zinc or zinc-aluminium coatings and may eventually replace them in some areas. They are not a serious competitor to galvanized coatings, which cover significantly longer protection periods due to their significantly higher layer thicknesses.
When the spray is galvanized, also called thermal zinc spray, the zinc is melted. For example, sprayed from a spray gun on the surface of the steel part. It is regulated in the DIN EN ISO 2063 standard. The thickness of "zinc plated" zinc coatings is generally between 80 and 150 microns. Spray galvanized coatings must be additionally sealed by a penetrating coating.
With regard to the duration of protection of galvanized coatings, there are no uniform standard specifications, as according to DIN EN ISO 14713, arc spray is most commonly used as a corrosion protection system with sealing later, the corrosion protection of the system will largely depend on the correct version.
Galvanizing is often used as corrosion protection for steel parts, which cannot be galvanized by design. In contrast to hot dip galvanizing, zinc plating and steel do not form an alloy during spray galvanizing. Spray galvanizing is regulated in ISO 2063.
In galvanic zinc, zinc is deposited on steel parts by means of electric current. Electrolytes are generally acidic, but also cyanide-free or cyanide-free alkaline. The resulting zinc coatings are comparatively thin and are mostly at 5 microns, even if greater layer thicknesses are possible. Galvanizing is therefore primarily used for temporary corrosion protection tasks in mildly corrosive environments. In contrast to other galvanizing processes such as hot dip galvanizing, galvanizing does not lead to alloying between the zinc coating and the steel. Zinc coating consists of a pure zinc coating.
Galvanizing takes place discontinuously according to DIN 50979 or continuously DIN EN 10152 + 10244-2. Layer thicknesses of discontinuously applied galvanic zinc coatings are generally between 5 and 25 micrometers.
One of the most well-known galvanized products is the bright silver ones, which have a reddish rust after a few years when used outdoors and are not to be confused with galvanized screws.
NO GALVANIZATION (ZINC POWDER COATING, ZINC FLAKE COATING)
Zinc powder coatings are also used to repair defects in hot dip galvanized coatings. Coating systems containing zinc, such as zinc dust and zinc flake coatings, are not among the galvanizing processes, although they are often associated with the term "galvanizing". The common element of all galvanizing is the application of a metallic coating that consists essentially of zinc. Coating systems containing zinc do not meet this criterion.